This invention relates to a weldable ferritic stainless steel having good fabrication characteristics. More particularly, the invention relates to a weldable corrosion resistant ferritic stainless steel suitable for forming integrally-finned tubular articles.
There are numerous applications for finned tubular products having increased surface area for increasing the heat transfer efficiency of the tubing for condensers, heat exchangers, evaporators, reheaters, and the like. Though aluminum, copper and plain carbon steels are frequently used for such applications, Type 304 austenitic stainless steel having nominally 18% chromium and 8% nickel has not found favorable use due to its poor resistance to stress corrosion cracking.
Though ferritic stainless steels offer desirable properties of resistance to general corrosion, as well as stress corrosion cracking, they have not become popular because of poorer mechanical properties and fabricability. Efforts have been made to improve the formability of ferritic stainless steels such as disclosed in U.S. Pat. No. 3,607,237, issued Sept. 21, 1971, and U.S. Pat. No. 3,607,246, issued Sept. 21, 1971, by limiting the carbon content and including small additions of titanium to improve formability. Such alloys are suitable for manufacturing processes including high-speed punching presses involving stamping, punching, piercing, blanking and drawing.
A ferritic stainless steel useful in moderate corrosion environments is disclosed in U.S. Pat. No. 3,850,703, issued Nov. 26, 1974, having sufficient ductility to be cold rolled direct final gauge from hot band. The steel includes aluminum to provide adequate weldability and titanium for formability. A ferritic stainless steel is disclosed in U.S. Pat. No. 3,953,201, issued Apr. 27, 1976, having good corrosion resistance, low yield strength, low tensile strength and good ductility by controlling element additions and residuals.
Recent developments in melting techniques have made it possible to produce ferritic stainless steel, such as Type 439, which has been used with beneficial results when compared to Type 304 austenitic stainless steel. Type 439 is a titanium and/or columbium stabilized ferritic stainless steel having a nominal chemistry of up to 0.07 carbon, 0.1-0.6 manganese, 0.2-0.6 silicon, 17.75-18.75 chromium, up to 0.5 nickel and up to 0.15 aluminum and the balance essentially iron with usual steel-making residuals. That steel has a lower alloying content than Type 304 and can be used to manufacture integrally-finned tubing having a good general corrosion resistance as well as good pitting and crevice corrosion resistance in chloride environments. Particularly, carbon, nitrogen and titanium are controlled such that the total carbon plus nitrogen is less than 0.04 and the titanium ranges from a minimum of 0.2 plus four times the total carbon plus nitrogen content to a maximum of 0.85%.
Though Type 439 alloy has provided some success in improved mechanical properties and fabricability for use in integrally-finned tubing in moderate to severe corrosion environments, there is still a need for a ferritic stainless steel suitable for fabrication into tubing having increased efficiencies such as are needed for MSR (Moisture Separator Reheaters) applications in power plants. Such a ferritic stainless steel alloy should be compatible with such systems and provide improved ductility to permit the fabrication of increased fin height for good to excellent heat transfer characteristics while substantially eliminating any microcracking of fins as a result of fabrication. It is desirable that the alloy be stabilized to minimize formation of carbide and nitride particles to reduce die near during forming, as well as to substantially reduce microcracking of the fins. It is also desirable that the alloy have a lower alloying content to lower the cost of manufacture of the alloy.